1. Field of the Invention
This invention relates to a method and device for information anti-duplication for performing anti-duplication control corresponding to an anti-duplication control signal transmitted and received together with an information signal in the case that an information for preventing duplication of a video signal played back from a recording medium is outputted and received together with a video signal in order to limit or inhibit recording of the video signal on another recording medium.
2. Description of the Related Art
VTR (Video Tape recording devices) has been popularized in daily life, and many kinds of software which can be played back on a VTR are supplied abundantly. Digital VTR or DVD (Digital Video Disks) playback devices have been available practically now, and provide images and sound of exceptionally high quality.
On the other hand, there is, however, a problem in that this great abundance of software can be copied without restriction, and several methods have already been proposed to inhibit duplication.
For example, for a VTR which outputs an analog video signal, one method to prevent copying uses a difference in the AGC (Automatic Gain Control) system, or in the APC (Automatic Phase Control) system, of the VTR recording device and of a monitor receiver for displaying the image.
When the method, in which the difference in AGC system is utilized and a VTR performs AGC using a pseudo sync signal inserted in the video signal and a monitor receiver employs a different AGC system not using the pseudo sync signal, is used, a very high level pseudo sync signal is inserted in the video signal supplied from the playback VTR and the video signal with insertion is outputted to the recording VTR as an AGC sync signal.
When the method in which the difference in APC characteristics is utilized, and a VTR performs APC using the phase of the color burst itself in the video signal and a monitor receiver employs an APC system different from that of the VTR, is used, the phase of the color burst of the video signal supplied from the playback VTR to the recording VTR is inverted partially.
As the result, the monitor receiver which receives the analog video signal from the playback VTR plays back the image correctly without being affected by the pseudo sync signal in AGC or the partial phase inversion of the color burst signal used for APC.
On the other hand, when a VTR records, on a recording medium, the analog video signal into which pseudo sync signals have been inserted or which has been subjected to color burst signal phase inverting control in the playback VTR as described herein above, proper gain control or phase control based on the input signal cannot be performed, and so the video signal is not correctly recorded. Even if this signal is played back, therefore, normal picture and sound cannot be obtained.
In the case of a digitized video signal, for example, in a digital VTR, an anti-duplication signal or an anti-duplication control signal comprising, for example, a duplication ranking control code, is added as digital data to the video signal and recorded on the recording medium, so as to prevent or control duplication.
In this case, the playback digital VTR reads the video signal, audio signal and anti-duplication control signal, and supplies them as digital or analog data to a recording digital VTR.
In the digital VTR being used as a recording device, the anti-duplication control signal is extracted from the supplied playback signal, and recording of the playback signal is then controlled based on the anti-duplication control signal. For example, when the anti-duplication control signal comprises an anti-duplication signal, the recording VTR does not perform recording.
Alternatively, when the anti-duplication control signal comprises a copy ranking control code, recording is controlled by this ranking control code. For example, when the copy ranking code limits duplication to one copy, the digital VTR used for recording adds this anti-duplication code before recording the video signal and audio signal on the recording medium as digital data. It is thereafter impossible to duplicate the video signal from the copy.
Hence, in the case of a digital connection when the video signal, the audio signal, and the anti-duplication control signal used as digital signals are supplied to the digital VTR used as a recording device, anti-duplication control is performed on the recording side using the anti-duplication control signal by supplying this signal to the digital VTR as digital data.
However, in the case of analog connection where the video signal and audio signal are supplied as analog signals, D/A conversion of the signal supplied to a recording device causes the loss of the anti-duplication control signal. Hence, in the case of an analog connection, an D/A converted anti-duplication control signal must be added to the D/A converted video signal or audio signal, and such addition results in deteriorated video signal or audio signal.
It is, therefore, difficult to add a D/A converted anti-duplication control signal and to extract it in the recording device for the purpose of anti-duplication control, without causing deterioration of the D/A converted video signal or audio signal.
Conventionally, therefore, in the case of an analog connection, duplication was prevented by an anti-duplication method using a difference in AGC, or a difference in APC characteristics, between the VTR and the monitor receiver.
However, in some cases, when anti-duplication is prevented using the above-mentioned difference in AGC or a difference in APC characteristics between the VTR and the monitor receiver, depending on the type of AGC or APC characteristics on the recording side, the video signal may nevertheless be correctly recorded, in this case, it might happen that duplication cannot be prevented, or that the played back image on the monitor receiver is distorted. Further, it was troublesome to change over the anti-duplication method depending on whether there is an analog connection or a digital connection.
To solve such problem, the inventors of the present invention previously proposed the method that spectrally spread anti-duplication control signal is superimposed on the video signal (Japanese Patent Application No. Hei 7-39959) as an anti-duplication method which is effective for both analog connection and digital connection without causing deterioration of played back image and sound.
According to this method, in the playback device side, a PN (Pseudorandom Noise) sequence code (referred to hereinafter as PN code) used as a spread code is generated frequently with a sufficiently short period and spectrally spread by multiplying it by the anti-duplication control signal. In this way, a narrow bandwidth, high level anti-duplication control signal is converted to a wide band, low level signal which does not affect the video signal or sound signal. This spectrally spread anti-duplication control signal is then superimposed on the video signal supplied to the recording medium, and outputted.
On the other hand, in the recording side, a PN code having the same timing and phase as the PN code used for spectral spreading in the playback device relative to the video signal supplied by the playback device is generated, and the generated PN code is multiplied by the video signal on which the anti-duplication control signal is superimposed so that the original anti-duplication control signal is extracted, that is, so that inversion spectral spreading is performed. Anti-duplication is then controlled based on the anti-duplication control signal extracted by inversion spectral spreading.
In this way, the anti-duplication control signal is spectrally spread and superimposed on the video signal as a wide band, low level signal in the playback device. It is therefore difficult for a person who wishes to illegally duplicate the video signal, to remove the anti-duplication control signal which is superimposed on it.
However, it is possible to detect and use the superimposed anti-duplication control signal by inversion spectral spreading. This anti-duplication control signal is therefore supplied to the recording device together with the video signal. In the recording side, the anti-duplication control signal is detected, and duplication is consistently controlled according to the detected anti-duplication control signal.
According to this method, as described herein above, the spectrally spread anti-duplication control signal is superimposed as a wide band, low level signal on the video signal, but it must be superimposed at a lower S/N ratio than that of the video signal in order for the video signal not to cause deterioration of the video signal.
To superimpose the spectrally spread anti-duplication control signal at a lower S/N ratio than that of the video signal, and to be able to detect the anti-duplication control signal superimposed on the video signal in the recording device, the number of the PN codes (PN code length) required to spectrally spread a one bit anti-duplication control signal must be sufficiently large. The PN code length per bit of the anti-duplication control signal may also be expressed as a spread gain (spread factor) which is the ratio (T/TC) of a time width T per bit of the anti-duplication control signal and a time width TC of one part (one chip) of the PN code. As described hereinafter, this spread gain is found from the S/N ratio of the information signal on which the anti-duplication control signal is superimposed, in this case the S/N ratio of the video signal.
For example, when the S/N ratio of the video signal on which the anti-duplication control signal is superimposed is 50 dB, the anti-duplication control signal which is spectrally spread and superimposed on the video signal must be superimposed at a lower level than 50 dB, which is the S/N ratio of the video signal. Also, in order to detect the anti-duplication control signal superimposed on the video signal, its S/N ratio must be sufficient for the spectrally spread signal to be fully demodulated. If this S/N ratio is 10 dB, a spread gain of 60 dB ((S/N ratio of 50 dB for video signal)+(S/N ratio of 10 dB necessary for detection)) is required. In this case, the PN code length per bit of the anti-duplication control signal is 1 million code length.
The method used in the recording device to detect the PN code superimposed on the video signal uses a matched filter or a sliding correlation. In the former case, the PN code is detected quickly, but only a short code length can be detected. At present, this code length is of the order of 256, and when the PN code length is 1 million per bit of the anti-duplication control signal, it cannot be detected. In the latter case, PN codes of long length can be detected but the detection takes a long time. It can thus be anticipated that a considerable time is required to detect a PN code having a length of 1 million.
Moreover, in the case that the spectrally spread anti-duplication control signal is superimposed on a video signal and supplied to a recording device using a PN code having a code length which can not be detected by means of a matched filter, it takes a long time for detecting the anti-duplication control signal in the recording device side, the long time for detection is inconvenient.
An anti-duplication method as described herein under is considered effective in the case of a recording device which duplicates a video signal on which a spectrally spread anti-duplication control signal is superimposed. The recording device starts first recording processing regardless of an anti-duplication control signal, and simultaneously detects the anti-duplication control signal superimposed on the video signal, after detecting the anti-duplication control signal, duplication control is performed based on the detected anti-duplication control signal.
In this case, it is possible to record the video signal normally at least until the anti-duplication control signal is detected. As the result, the initial portion of the video signal is undesirably duplicated though the anti-duplication control signal is a signal to inhibit duplication. Therefore, in the case that detection of the anti-duplication control signal requires several seconds to several ten seconds, it is possible to duplicate a whole video signal by repeating recording processing a plurality of times though the video signal is inhibited to be duplicated.
To solve such problem, it is considered conversely that first an anti-duplication control signal is detected and then duplication control is performed based on the detected anti-duplication control signal.
However, in this case, though there is no problem when the anti-duplication control signal is a signal for inhibiting duplication of the video signal, but when the anti-duplication control signal is a signal for permitting duplication of the video signal, recording is not operated until the anti-duplication control signal is detected, hence the initial portion of the video signal to be recorded normally is not recorded.
Not only in the case that an anti-duplication control signal on which a spectrally spread video signal is superimposed is detected for duplication control as described herein above but also, for example, in the case that an anti-duplication control signal added to a video signal is extracted in the recording device and duplication control is performed depending on the extracted anti-duplication control signal, undesirable recording of the initial portion of the video signal for inhibited duplication and unpreferable blank recording of the initial portion of the video signal for permitted duplication are problems.
In view of the above-mentioned problems, it is the object of the present invention to provide a method and device for information duplication prevention which are capable of eliminating such problems and performing duplication control consistently and adequately.